DESCRIPTION

The EVP_PKEY_keygen_init() function initializes a public key algorithm
context using key pkey for a key genration operation.

The EVP_PKEY_keygen() function performs a key generation operation, the
generated key is written to ppkey.

The functions EVP_PKEY_paramgen_init() and EVP_PKEY_paramgen() are similar
except parameters are generated.

The function EVP_PKEY_set_cb() sets the key or parameter generation callback
to cb. The function EVP_PKEY_CTX_get_cb() returns the key or parameter
generation callback.

The function EVP_PKEY_CTX_get_keygen_info() returns parameters associated
with the generation operation. If idx is -1 the total number of
parameters available is returned. Any non negative value returns the value of
that parameter. EVP_PKEY_CTX_gen_keygen_info() with a non-negative value for
idx should only be called within the generation callback.

If the callback returns 0 then the key genration operation is aborted and an
error occurs. This might occur during a time consuming operation where
a user clicks on a ``cancel'' button.

The functions EVP_PKEY_CTX_set_app_data() and EVP_PKEY_CTX_get_app_data() set
and retrieve an opaque pointer. This can be used to set some application
defined value which can be retrieved in the callback: for example a handle
which is used to update a ``progress dialog''.

NOTES

After the call to EVP_PKEY_keygen_init() or EVP_PKEY_paramgen_init() algorithm
specific control operations can be performed to set any appropriate parameters
for the operation.

The functions EVP_PKEY_keygen() and EVP_PKEY_paramgen() can be called more than
once on the same context if several operations are performed using the same
parameters.

The meaning of the parameters passed to the callback will depend on the
algorithm and the specifiic implementation of the algorithm. Some might not
give any useful information at all during key or parameter generation. Others
might not even call the callback.

The operation performed by key or parameter generation depends on the algorithm
used. In some cases (e.g. EC with a supplied named curve) the ``generation''
option merely sets the appropriate fields in an EVP_PKEY structure.

In OpenSSL an EVP_PKEY structure containing a private key also contains the
public key components and parameters (if any). An OpenSSL private key is
equivalent to what some libraries call a ``key pair''. A private key can be used
in functions which require the use of a public key or parameters.

RETURN VALUES

EVP_PKEY_keygen_init(), EVP_PKEY_paramgen_init(), EVP_PKEY_keygen() and
EVP_PKEY_paramgen() return 1 for success and 0 or a negative value for failure.
In particular a return value of -2 indicates the operation is not supported by
the public key algorithm.